1. Field of the Invention
The present invention relates to a tracking control method and apparatus for controlling capstan speed by using a variable capstan speed instruction in the image recorder/reproducer, wherein the amount of tracking error is reflected, thereby carrying out correct tracking.
2. Discussion of Related Art
An image recorder/reproducer such as a VCR or a video camera/recorder (camcorder) generates a poor display when beat frequencies are not properly controlled by tracking. Therefore, good tracking control is essential to obtain a good screen image.
Tracking control operation involves controlling the head correctly to scan a predetermined track, thereby maximizing envelope output during reproduction.
A separate control signal is used for tracking control. In a VHS-VCR, tracking is performed using a control signal which is recorded on a control track of a respective tape by a fixed head. Further, in an 8 mm VCR system, the pilot signal used in automatic track finding (ATF), has four different frequencies, and is recorded on the track of the tape. The pilot signal is used for reproduction in such a way that the tracking error, i.e., the degree of deviation from the track which the current head is scanning, is detected and reduced.
It will be noted that, the detected tracking error can be minimized by controlling the spinning speed of the capstan motor. That is, the object of the tracking control is to determine the optimum spinning speed of a capstan motor.
A conventional tracking control method will be described with reference to the attached drawings.
FIG. 1A and FIG. 1B show the relation between the scanning traces of the head and tracking error of a conventional VCR. For convenience, the generation of a positive error indicates that the head is located prior to the track to be scanned while the generation of a negative error indicates that the head is located after the track is scanned.
Referring to FIGS. 1A and 1B, when the head is located prior to the predetermined track, that is, when the head is located in a P1-level track, the tape proceeds slowly by slowing down the speed of the capstan motor. When the head is behind the predetermined track, that is, when the head is located in a P2-level track, the tape proceeds faster by increasing the speed of the capstan motor. Therefore, the head is controlled to scan the predetermined track correctly.
This basic principle of tracking control is still appropriate for the present invention as well as for the conventional method.
The conventional structure of a capstan servo for tracking control is shown in FIG. 2.
Referring to FIG. 2, the speed control signal for a capstan motor 20 applied to a capstan motor driver 18 is determined by the amount of capstan speed control detected from a tracking controller 12 and the amount of tracking control detected from a capstan controller 14, respectively. An adder 16 adds the above two amounts.
FIG. 3 shows an embodiment wherein the capstan servo system shown in FIG. 2 is applied to a VHS-VCR.
Here, the amount of capstan speed control is calculated under the assumption that there is a predetermined normal track, and the capstan speed instruction (f), which corresponds to the regular spinning speed of the capstan motor and which is needed when the head correctly runs above the normal track, is the reference value. Also, the reference value and the actual speed of a capstan motor 27, as measured by a frequency generator 29 when capstan motor 27 spins, are compared in a capstan speed error detector 21. The degree of error calculated from this comparison is changed into a degree of speed control of capstan motor 27 in a capstan speed controller 22, which is not affected by the degree of tracking control for compensating the tracking error.
The record pulse which is synchronized with the track start point (hereinafter, termed a `control pulse`) is recorded on the control track. Also, the degree of capstan tracking error is output from the capstan phase comparator 23 by means of phase comparison between the control pulse which is reproduced by a fixed head 28 (called a `control head`) and the head switching pulse wherein the prearranged frequency is provided. Then, the degree of capstan error is proportional-integration-differentiation (PID) controlled so as to be output as a degree of capstan tracking control, i.e., a predetermined physical amount, in a capstan phase controller 24, thereby performing phase control of a capstan motor in order to control tracking.
The degree of capstan speed control of capstan speed controller 22 and the degree of tracking control of capstan phase controller 24 are added in an adder 25, and the capstan motor 27 is driven by a capstan motor driver 26.
However, the above-mentioned conventional method generates various kinds of problems when the direction for controlling the speed of the capstan motor and the direction for controlling the tracking are different. The following is an explanation with reference to FIG. 2 and FIG. 4.
FIG. 4 shows the tracking when the head runs along a deviating track modified from the normal track.
As can be seen in FIG. 4, the traces of the head which capstan speed controller 14 aims to control are on the predetermined normal track (the A direction of FIG. 4), while the traces of the head which tracking controller 12 aims to control are on the modified track (the B direction of FIG. 4).
Accordingly, when the direction for controlling the speed of capstan motor 20 and the direction for controlling the tracking are different, correct tracking is difficult since the speed controller prevents the capstan motor 20 from moving in the direction of reducing the tracking error.
Precise gain control for the degree of control for the above two directions is needed to solve the problems. Therefore, the gains of tracking controller 12 and capstan speed controller 14 should be controlled precisely. However, the design of the device which is additionally required for the gain control as well as the gain control itself is difficult. Even though the device has a gain control function, there is still a problem of compatibility between the two devices if the recording device and the reproduction device are different.
The deterioration in tracking is not a significant problem in a VHS-VCR or in an 8 mm VCR, since the width of the track is quite large (58 .mu.m for VHS, and 21 .mu.m for 8 mm). However, there is a problem of lowered performance for the case of digital VCR which requires high density recording, because very high performance tracking is needed if the track width is below 10 .mu.m.